Yeah, I didn't say anything about your suggestion that Die Antwoord could headline the second stage.
Having said that, though, I think they'll definitely have someone more typically "Download" than Die Antwoord on at the same time as Tool.

Well, alternatively, they could have them on in that slot to give prog fans something to watch instead of Smashing Pumpkins in order to further entice them to buy day tickets.
If it's not Dream Theatre headlining second I don't really see why the stage would close early. I definitely think they'll want someone up against Tool on the second stage - either a straight up metal band or something for a younger audience (like a pop-punk band or Enter Shikari or something).

There have been countless stories about this exact thing recently, and established artists have been pointing it out, too (Lily Allen, for example). I don't really care enough about educating you on the matter to go off and find these examples, so I won't.
It's quite clear that opportunities and support for male acts are much bigger than they are for female acts. How many female acts make it to the higher reaches of festival lineups? Not many. How many female-fronted bands are touted as the next big thing in the same way the likes of The 1975, Royal Blood, Greta Van Fleet, etc. are? Basically none. How many new female rappers are touted as the next big thing? Barely any. Are the female acts worse than the male ones? No, they aren't. Is it the case that your stereotypical huge rock band or huge rapper is a male? Yeah, which is why these male acts get backed as the next big thing over their female counterparts. How does that change? We change the perception that your giant rock bands/rappers don't have to be male, and that females can occupy those spaces just as well.
Right now, a young boy wanting to start playing the drums or electric guitar will be given more encouragement to do that than a girl of the same age, when they see that pretty much all the people who make it to the big time in those roles are males. Is there something inherently male about playing the drums or electric guitar? No, there isn't, but the culture around rock music paints them as male hobbies.
I understand what you're saying about it being unfair to book an act based on their gender. But I think it's more unfair that the industry is like this and hence females are afforded less opportunities to make it. And unfortunately, we're in a position where measures like this have to be taken to address the imbalance.
Anyway, I'm pretty bored of going over this argument for the thousandth time on here now so I'm gonna leave it there.